SPECTRAL DIFFUSION IN ORGANIC GLASSES - TIME-DEPENDENCE OF SPECTRAL HOLES

Time-resolved spectral hole-burning experiments have been performed to probe the dynamics of the S-1 --> S-0 0-0 transition of bacteriochlor ophyll-a at low concentration (1 x 10(-5)M) in four different glasses (2-methyltetrahydrofuran, protonated and deuterated ethanol, diethyl e ther, and triethylamine) as a function of delay time t(d) (from 10(-6) to 10(3) s) and temperature T (1.2-4.2 K). It is shown that spectral diffusion, the broadening of the optical linewidth followed here over nine orders of magnitude in time, increases with temperature as T-1.3/-0.1 and strongly depends on the glass structure. The functional depe ndence, however, is not influenced by the specific glass. The variatio n of the ''effective'' homogeneous linewidth (Gamma(hom)') with T and td is described by a function Gamma(hom)'(T,t(d)) derived by modifying the standard model of two-level systems (TLS). This revised TLS model , in which the distribution functions of the TLS tunneling parameters are different from those in the standard model, takes into account the common origin of the dependence of Gamma(hom)' on t(d) and T. It is s hown that other hole-burning and photon-echo data reported in the lite rature can also be fitted by the same function Gamma(hom)'(T,t(d)). In ethanol glass, the number of TLSs and the amount of spectral diffusio n appear to be independent of the probe molecule.